The present invention relates to a wire bonder.
In an existing wire bonder for assembling a semiconductor device, one worker usually takes care of one wire bonder and does his bonding work. As a result, the disconnection of a wire (i.e., a gold wire) can be confirmed, when it occurs, by the worker so that he can instantly stop the bonder to apply the wire to a predetermined position.
However, when the wire bonding operation is automated so that the worker takes care of several bonders, it is indispensable to automatically detect the disconnection of the wire, instantly when it takes place, so that the troubled bonder may be stopped.
When in the wire bonding oeration, care has to be taken for prevention of a variety of accidents such as pellet short-circuiting resulting from contact between the wire and a pellet, table short-circuiting resulting from contact between the wire and a table, or wire short-circuiting resulting from mutual contact between the wires. Moreover, even if the semiconductor device is operated so that it is heated or cooled, the wire has to be prevented from being disconnected.
It is, therefore, necessary to allow the wire (i.e., the gold wire), which is tensed between the semiconductor pellet and its lead, to draw a smooth curve. There has been provided according to the prior art means for drawing such curve, which is effected by bonding a first portion to be bonded, by subsequently letting off a wire having a larger length than a required one from a capillary while the capillary is being located around for a second bonding step, and by bonding a second portion to be bonded while slightly taking up again that wire upon its spool. In view of these circumstances, a torque is so imparted to the spool having the wire wound thereon as to allow the spool to rotate backward at all times, whereas a back tension is applied to the wire at all times.
Once the wire is cut, therefore, it is abruptly taken up by the action of the rotating torque of the spool. The wire is randomly wound, if it is taken up in that way, upon the spool, and its end portion is waved about to contact with the surrounding member or members so that it is damaged. The wire thus damaged is frequently entangled, cut or brought into a substantially cut state. This raises disadvantages that it takes a long time to prepare the bonding operation and that the semiconductor device fabricated is troubled while it is being used.
In order to eliminate these disadvantages concomitant with the prior art, there exists a wire bonder of thermo compression neil head wire bonding type, as is disclosed in Japanese patent publication No. 53-31713, in which whether or not the wire is disconnected is detected to control in case of the disconnection the torque applied to the spool so as to tense backward the wire.
FIG. 1A is a perspective view showing a major portion of the above-specified wire bonder according to the prior art. Reference numeral 101 indicates a hollow spool holder having an outer circumference, on which a spool 102 is rotatably fitted. The spool holder 101 is made of such a cylinder as is formed with a number of air-injecting through holes at a certain angle between the inside and outer circumference thereof. The openings of those injecting holes are so directed that the spool 102 is rotated in the opposite direction to the wire letting-off direction indicated at an arrow. As a result, if compressed air is supplied into the center bore of the spool holder 101, it is injected from those injecting holes thereby partly to bear the spool 102 with the air injected and partly to impart a backward torque to the spool 102.
In FIG. 1A: numerals 105, 106, 107, 108, 109, 110 and 111 indicate a clamper, a capillary, a wire disconnection detecting circuit, a timing pulse supply circuit, a flip-flop circuit, a valve actuator and an electromagnetic valve, respectively.
The detail of the clamper 105 is shown in FIG. 1B. On the inner sides of clamping arms 112 and 113, there are borne insulating members 114 and 115, one of which 114 is equipped with an electrode 116. The clamper 105 thus constructed is actuated by means of an electromagnet or a cam, when in the bonding operation, to clamp a wire W.
The aforementioned electrode 116 is used to contact with the wire W thereby to confirm the existence of it and is connected through a lead wire 117 with the wire disconnection detecting circuit 107.
Next, the operations of the system thus constructed will be described in the following. After a ball W.sub.1 formed at the leading end of the wire W is forced to contact with the pad portion on a semiconductor pellet 119 secured to a table 118, the capillary 106 is moved in the direction of arrow A of FIG. 1A to force the wire W to contact with a lead 120 until the state of FIG. 1C is established. Then, the clamper 105 and the capillary 106 are elevated, as indicated at arrow B, to cut the wire W at a position close to the lead 120.
As shown in FIG. 1A, when the capillary 106 tenses the wire W from a first bonding portion (i.e., the pad portion on the pellet) to a second bonding portion (i.e., the lead), the clamper 105 is left open. Since, at this state, the voltage at a point D of the detecting circuit 107 is high, the transistor is rendered conducting so that the potential at a point C is at substantially the same level of 0 V as that of the earth terminal.
Next, when the lead 120 is to be bonded, as shown in FIG. 1C, the clamper 105 is closed, and the potential at the point D of the detecting circuit is grounded to the earth through the electrode 116, the wire W and the lead 120 so that it is reduced to 0 V so that the transistor is cut off. As a result, the potential at the point C becomes substantially equal to the voltage of the power source.
In case the wire W is cut between the semiconductor pellet 119 and the lead 120, as shown in FIG. 1D, the electrode 116 is not grounded so that the potential at the point D fails to take 0 V. As a result, the transistor is rendered conducting so that the potential at the point C is reduced to 0 V.
It is measured by the aforementioned method whether or not the wire W is disconnected. In this method, it is sufficient that the potential at the point C is measured when timing pulses for closing the clamper 105 are received during the bonding operation. In short, it is indicated that the wire W is not cut, if the potential at the point C is high, but is cut if that potential is low.
If the existence of the disconnection of the wire W is detected in the aforementioned manner by the use of the clamper 105, the flip-flop circuit 108 is set, when the pulses are supplied from the timing pulses supply circuit 108, and the valve actuator 110 is operated in response to the signal of the flip-flop circuit 109 to close the electromagnetic valve 111 thereby to interupt the supply of the compressed air, which has been continued to the holes of the spool holder 101 by way of a conduit 121 until that time.
As has been described hereinbefore, the wire bonder is so constructed to detect whether or not the wire is disconnected that the supply of the compressed air for reversing the spool is controlled by the detection signal. As a result, as soon as the disconnection takes place, the detecting unit operates to prevent the spool from rotating backward.
As a result, since the wire is not wound randomly upon the spool, the wire can be let off again in a smooth manner so that the trouble in the wire bonding operation such as the disconnection of the wire can be prevented.
However, the wire bonder thus far described according, to the prior art is equipped with the means for detecting the disconnection of the bonding wire. In spite of this equipment, that detecting means may fail to operate at a normal state so that the bonding wire itself of the wire bonder may be disconnected before it is bonded to a work such as the pellet or the lead. If the wire bonder is operated while having its bonding wire left disconnected, it frequently occurs that the bonding wire cannot be applied to the portion of the work to be bonded. This will be described in the following.
In the disconnection detecting means of that type according to the prior art, a clamper to clamp the bonding wire is made of a conductive material to provide conduction between the wire and the clamper during the clamping operation, and the clamper is connected with one electrode whereas the wire is connected with the other electrode at the side of the semiconductor pellet or the spool thereby to form the clamper and the bonding wire into a part of the predetermined detecting circuit. In the means having the construction thus far described, in the normal case in which the wire is not disconnected, the clamper clamps the wire to provide conduction inbetween. When the wire is disconnected, on the other hand, the clamper releases the wire and fails to clamp it any more so that the portion of the aforementioned detecting circuit is at its open state. As a result, it becomes possible to detect the disconnection of the wire in accordance with the conduction and inconduction between the clamper and the wire.
However, it has been found difficult for that means to accurately detect especially the disconnection of the wire bonder of ultrasonic type using an aluminum wire as the bonding wire. The intense investigation of the cause for that difficulty has revealed that, since the aluminum wire has its surface formed with alumina having bad electric conductivity, the clamper and the aluminum wire are insulated, even if this aluminum wire is normally clamped, by the alumina formed on the surface of that aluminum wire so that the detecting circuit is not closed to invite a malfunction in the detection. It is also found that in the case of such malfunction an automatic interruptor operates, in spite of the normal operation of the wire bonder, to invite an emergency stop of the wire bonder, and that this difficulty remarkably deteriorates the working efficiency.